A critical overview of measurement techniques of spacecraft charging in space plasma

Abstract

Under certain conditions in space there are physical processes which may cause misinterpretation of spacecraft potential measurements using some very common techniques. The use of long booms to measure the potential difference between a spacecraft and the tip of a boom has at least two deficiencies. First, when the spacecraft potential is high, the tip of the boom may be engulfed by the sheath of the spacecraft. Second, the tip of the boom itself may charge. Even if materials of high secondary emission coefficients are used for the boom tips, charging can still occur when the space plasma is energetic. Another common technique for measuring the energy shift of the incoming electron energy distribution function also has deficiencies. First, if ionization is abundant in the vicinity of the spacecraft (for example, during electron beam emissions), the energy gap resulting from the shift becomes blurred. Second, if the angular momentum of the incoming electrons is significant, the shift should not be attributed to potential energy only. The technique of using Langmuir probes, which are located at short distances from the spacecraft surface, is often deficient in measuring spacecraft potential. It may be too naive to apply either the attractive regime or the repulsive regime of Langmuir. For example, while the probe potential may be negative relative to that of the space plasma, it may be positive relative to that of the spacecraft. In that case, the probe is repelling electrons from the space plasma but attracting secondary electrons from the spacecraft.